Fuel cell systems are used, inter alia, as auxiliary power units (APUs) for generating electrical energy on board motor vehicles. Hydrogen and carbon monoxide serve primarily as fuels that can be supplied to the anodes of the fuel cell stack, wherein these fuel gases are generated by partial oxidation of a conventional fuel (e.g., diesel or petrol) in a reformer connected upstream of the fuel cell stack.
A series of known problems exists in the operation of fuel cell systems of this type, which can lead to a reduction in the efficiency and/or the operating life of the fuel cell stack or make additional measures necessary which themselves worsen the cost/output ratio of the system. These problems arise essentially from the composition of the available fuels and the reformate resulting therefrom and concern, above all (though not exclusively) the start and end phase of the operation of the fuel cell system, in which the fuel cell stack is not yet or is no longer at its optimum operating temperature. In the typically-used high temperature fuel cells, this lies in the region above 700° C.
A general problem is the sulfur content of the fuels which in commercially available diesel fuel, despite the designation “sulfur free”, is up to 10 ppm. Although this quantity is still tolerable at an operating temperature of the fuel cell stack of more than 900° C., in the temperature range from 700° C. to 800° C., which is preferable from the standpoint of efficiency, it leads over time to de-activation of the catalyst (particularly nickel) used at the anode, so that carbon monoxide, in particular, is no longer converted (sulfur poisoning). As a countermeasure, a desulfurization unit, which naturally leads to increased costs and an increased need for space, can be connected upstream.
During the start phase of the fuel cell system, the problem arises, firstly, that the fuel cell stack must be heated to the desired operating temperature, which is achieved by means of the pre-heated oxidizing agent (atmospheric oxygen) and possibly the hot reformate whilst, secondly, this reformate has an unfavorable composition for as long as the optimum operating temperature of the reformer has not been reached. Particularly in the range of approximately 300° C. to 600° C., the deposition of soot takes place when the hot reformate makes contact with the still cold anode material, which can lead in the long term to damaging of the anodes. However, heating of the fuel cell stack solely by means of the oxidizing agent is also disadvantageous because the temperature differences arising therefrom between the cathodes and the anodes can lead to thermal stresses and crack formation and because the oxidizing agent can then reach the anodes by way of a downstream residual gas burner. An oxidizing atmosphere at the anodes leads to oxidation of the catalyst (e.g., from nickel to nickel oxide) which, although it is in principle reversible, can however lead to damaging of the anode material.